Polycrystalline silicon and particularly in-situ doped polycrystalline silicon has been suggested as a contact material especially for ultralarge scale integration technology. Doped polycrystalline silicon has been suggested as a suitable plug or stud material (for a contact-hole or via) that makes contact with an underlying epitaxial monocrystalline silicon substrate. The silicon can be co-deposited on the desired substrate along with the dopant by chemical vapor deposition process. It has been reported that relatively deep-submicron contact holes have been successfully plugged with polysilicon.
Continuing efforts have been underway for providing improved methods for depositing doped polycrystalline silicon especially for improving the deposition rates and controlling radial non-uniformity across the wafer that has been caused by adding the dopant gas. Notwithstanding the strides that have been made, room still exists for improvement. This is especially so with respect to attempting to further reduce the contact resistance.
For instance, in-situ phosphorus-doped polycrystalline silicon exhibits contact resistance of about two-three times lower than a polycrystalline silicon doped after deposition with phosphorus. However, in-situ doping of phosphorus and boron requires fine-tuning of the dopant injection system and obtaining uniform in-situ doping is problematic. The problem of uniformity becomes significantly more acute when attempting to carry out in-situ doping of arsenic. Arsenic is a very desirable dopant because of its low diffusivity. This uniformity problem has been addressed to some extent by employing furnaces having loadlocks and a relatively complex array of injectors.
Furthermore, the deposition rate decreases considerably during phosphorus in-situ doping or arsenic in-situ doping.
Accordingly, providing an improved technique for achieving reduced contact resistance would be desirable.